In general, as means of using electricity have been increased, power supply has been further important. One of the representative means of using electricity is a vehicle. In general, electric power is used to start up the vehicle, and the electric power is also used for various apparatuses installed in the vehicle. Therefore, a vehicle battery is built in order to supply the power.
The vehicle battery is applied with a power to be charged in advance and has a function of outputting the charged power at a needed time. The vehicle battery has a limitation of a predetermined power. Therefore, if the charged power is completely discharged, the vehicle battery cannot output the power any more due to the discharging. Accordingly, it is inconvenient to maintain the vehicle battery in the state where the battery is always charged at a constant level or more.
As a general method of temporarily charging the aforementioned vehicle of which the battery is discharged, the battery is charged by applying the power output from the battery of another vehicle. However, in this case, the separate vehicle of which the battery is charged is needed, the bonnets of the two vehicles are opened, and the batteries of the two vehicles are connected by using a separate cable. Therefore, the charging process is very inconvenient. In order to solve the problem described above, a portable vehicle battery charger is used. However, there is a problem in that the aforementioned portable vehicle battery charger is useless during the period when the vehicle battery is normally charged.